imgur.com quite specifically does not work with codepen, because they block the codepen referer. You may still see the images in your browser if you’ve pulled up the imgur urls directly previously, due to them being cached. Everyone else will simply get a broken image (and you eventually will too).
I personally use google drive, some others here use dropbox. Just google for “direct link generator” plus the name of your preferred storage service. I’ve also heard good things about cloudinary.com, which is actually designed for this sort of thing.